At the Forefront of Solar Science

Wenda Cao, associate professor of physics and associate director of Big Bear Solar Observatory, at the 1.6-meter telescope dedicated to increasing knowledge of the Sun.

Four NJIT researchers presented intriguing recent discoveries about solar flares and sunspots at a press conference that was part of the 224th meeting of the American Astronomical Society (AAS) held in Boston, Massachusetts, in June. Invited to discuss their work were Dr. Santiago Vargas Dominguez, Dr. Alexander Kosovichev, Dr. Haimin Wang and Dr. Vasyl Yurchyshyn. NJIT colleagues also presented more than a dozen other papers during the AAS meeting that covered a broad range of topics in optical and radio solar astronomy.

The results described at the press conference were based on analysis of images obtained with the 1.6-meter telescope at Big Bear Solar Observatory (BBSO) in California in combination with data from NASA satellites. Operated by NJIT, the telescope at Big Bear is the most powerful ground-based optical instrument dedicated to studying the Sun.

Kosovichev is BBSO director, and Vargas Dominquez and Yurchyshyn are co-researchers at the observatory. Wang is director of the university’s Space Weather Research Laboratory, based on campus in Newark. Kosovichev and Wang are also members of the Physics Department faculty.

Discovering a Source of Solar Surges

Solar activity entails numerous processes occurring in the star nearest to Earth. These processes have far-reaching effects, generating “space weather” that sends bursts of charged particles and high-energy radiation in the direction of Earth at nearly the speed of light. On Earth, these solar storms can severely damage communications and power infrastructure.

In one of the three NJIT press-conference presentations, Vargas Dominguez reported on the emergence of buoyant "small-scale" magnetic-flux ropes on the solar surface and the initiation of powerful plasma eruptions in the solar atmosphere. The observations were performed as part of a program conducted jointly with NASA’s Interface Region Imaging Spectrograph (IRIS) mission, Solar Dynamics Observatory and Hinode satellite. They provided a unique view of a magnetic-flux rope in the Sun’s surface-granulation pattern that was 6,000 miles long, and the interaction between newly emergent and overlying ambient magnetic fields.

The magnetic field generated in the solar interior and brought to the surface creates a wide variety of structures, with sunspots being the most well-known. Sunspots can cover areas of the Sun’s surface many times the size of Earth. Associated with the evolution of sunspots, solar flares and coronal mass ejections are especially intense during the solar maximum, the period of greatest activity in the 11-year solar cycle.

The combination of ground- and space-based observations has facilitated investigation of how the layers of the solar atmosphere are linked, from the surface to the outermost layer, the corona. This has yielded important new understanding of solar activity and the mechanisms that drive it.

In particular, NJIT researchers have discovered previously unknown factors responsible for the generation of plasma surges and heating of the solar atmosphere. This research has shown that the complex action of small-scale and “hidden” fields on the Sun is important for understanding how energy is transported to the solar atmosphere. The process investigated can play a significant role in mass and energy flow from the Sun’s interior to the corona, the solar wind, and Earth’s near-space environment.

Solving Sunspot Mysteries

The dark regions on the left in this sunspot image are dense plasma sheets flowing into the sunspot from the solar atmosphere. The thin dark streaks on the right, near the center, are numerous plasma jets erupting from the sunspot. The very bright features also on the right are “Ellerman bombs,” areas where the sunspot’s magnetic energy is released in the form of impulsive explosions and heating.

The dark regions on the left in this sunspot image are dense plasma sheets flowing into the sunspot from the solar atmosphere. The thin dark streaks on the right, near the center, are numerous plasma jets erupting from the sunspot. The very bright features also on the right are “Ellerman bombs,” areas where the sunspot’s magnetic energy is released in the form of impulsive explosions and heating.

The presentation by Kosovichev and Yurchyshyn highlighted multi-wavelength observations of sunspots taken with the BBSO telescope and instrumentation aboard NASA's IRIS spacecraft. These observations have produced transformative views of high-speed plasma flows and eruptions extending from the Sun’s surface to the corona. High-definition video acquired at BBSO and shown at the press conference provides unique 3D views of a sunspot, revealing rapidly rotating plasma rolls, powerful shocks, and widespread plasma eruptions driven by solar-energy flux and controlled by intense magnetic fields. These leading-edge observations show that sunspots are far more complex and dynamic than previously believed.

Sunspots are still one of the greatest mysteries of astronomy. It has been known for more than a century that sunspots are compact, concentrated magnetic fields and that they appear dark because the magnetism prevents heat from rising to the surface from the superhot interior. But why these magnetic fields become so concentrated and compacted in structures that remain stable for days and sometimes weeks in a very turbulent environment is a mystery. There are no external forces on the Sun that could hold these giant magnetic structures together. They appear and are organized by their own induced forces.

Investigating sunspots is much more than a matter of increasing our fund of basic scientific knowledge. When sunspots that are close to each other have magnetic fields with opposite polarities, they can produce powerful flares and solar storms.

The high-definition video that Kosovichev and Yurchyshyn presented chronicles several hours in the life of an isolated sunspot that did not generate solar flares. But the roiling action revealed is an unprecedented view of sunspots as static-equilibrium structures maintaining a balance between magnetic force and gas pressure. The video captured small-scale activity of a generally “quiet” sunspot with detail never seen before. Remarkably, the organization of small-scale substructures is comparable to that observed at larger scales, indicating the existence of large-scale dynamics which control the formation and stability of sunspots.

Periodic pulses, or shocks, generated by sunspots comprise the most prominent feature of the Sun’s chromosphere, the middle of the three layers in the solar atmosphere. These shocks, which occur at intervals of about three minutes, travel into the high solar atmosphere at about 45,000 miles per hour and are observed by the IRIS spacecraft as ultraviolet flashes above the sunspot. The sunspot’s umbra, its darkest region, is covered by ubiquitous eruptions — plasma jets that may contribute to the shocks detected.

The most significant ultraviolet emissions and violent motion are observed above the area where the penumbra, or lighter region, intrudes into the sunspot’s umbra, the so-called “light bridge.” It is likely that this effect is related to anomalies in the sunspot’s magnetic topology, and requires further investigation. Some of the most dramatic events are high-speed plasma jets originating from the penumbra, as well as the apparent chromospheric accretion of dense plasma sheets into the sunspot. The origin of the accretion flows is another puzzle.

Investigating Three-ribbon Solar Flares

Images of two successive three-ribbon flares were captured with unprecedented resolution at Big Bear Solar Observatory on July 6, 2012. The ribbons are marked R1, R2 and R3.

The third NJIT press-conference presentation was given by Wang. He discussed an unusual type of solar flare — specifically, two successive three-ribbon solar flares observed in July 2012. The events were recorded by Wang’s co-author Wenda Cao, associate professor of physics and BBSO associate director. Flares with two ribbons are typical of these immensely powerful eruptions that can send storms of charged particles and high-energy radiation toward Earth.

The research Wang described integrated data acquired with the BBSO telescope and the Hinode satellite. The flaring site observed was characterized by an unusual "fish-bone-like" morphology evidenced by images acquired with an H-alpha filter and a nonlinear force-free field (NLFFF) extrapolation, where two semi-parallel rows of low-lying, sheared loops connected an elongated, parasitic negative field with sandwiching positive fields.

The NLFFF model also showed the two rows of loops to be asymmetric in height with opposite twists, and to be enveloped by large-scale field lines, including open fields. The two flares occurred in succession within half an hour and were located at the two ends of the flaring region. The three ribbons of each flare were parallel to the magnetic polarity inversion line, with the outer two lying in the positive field and the central one in the negative field.

Both flares showed surge-like flows in the H-alpha images presented by Wang, apparently toward the remote region. One of the flares also was accompanied by jets of extreme ultraviolet radiation, possibly along the open field lines. Interestingly, the 12-25 kiloelectron-volt hard X-ray sources of the flare first lined up with the central ribbon and then shifted to concentrate on top of the higher branch of loops. The results Wang discussed also suggest that reconnection — the interaction of newly emergent magnetic fields with pre-existing fields — along the coronal null line contributes to producing the three-flare ribbons and associated coronal mass ejections.

In addition to NJIT, funding for the research presented at the AAS press conference was provided by NASA, the National Science Foundation and the Air Force Office of Scientific Research.